The present invention relates to output voltage limiters for power supplies and the like and in particular to a low-voltage output limiter with high precision.
Electrical circuits normally operate in conjunction with a power supply delivering required voltage and current levels to the electrical circuitry. Particularly in safety and high reliability applications, it may be desirable that the power supply include output limiting circuitry for controlling output power from the power supply, for example, in the event of a short circuit across power supply output terminals or an increase in power supply voltage such as would cause the electrical circuit to consume additional power. In some applications, the outlet limiting circuitry must operate accurately at low voltages, for example, on the order of 3 to 4 volts (with 5% tolerance), while accommodating much higher voltages of up to 36 volts.
One way of providing such output limiting circuitry is to use a zener diode to control the biasing of a series transistor through which current must pass from the power supply to the electrical circuitry. As the voltage rises, the zener diode will limit the biasing of the series transistor thus decreasing the current flow between the power supply and electrical circuitry for protection. Such an approach normally requires that the zener diodes be screened and tested for low leakage current in order to provide such accurate level protection across a wide-temperature range required of industrial applications.
An alternative is to use a specialized integrated circuit that can monitor the output voltage of the power supply up to the highest expected voltage (e.g., 36 volts). Such integrated circuits may provide a precise comparator producing a switched output at a desired limited voltage. The switched output can then control a series transistor in place of the zener diode. The problem to this approach is that such integrated circuits that can tolerate high operating voltages are expensive